The source of progesterone in rabbit blastocysts.

High levels of progesterone were found in the blastocysts, uterine fluid and sera of normal pregnant rabbits on Days 6 and 7 of pregnancy. Very little progesterone (less than 2% of control) was present in bilaterally ovariectomized rabbits in which pregnancy was maintained by the administration of medroxyprogesterone acetate. The results demonstrate that the rabbit blastocyst is provided with large amounts of progesterone derived from the maternal ovaries.